1. FIELD OF THE INVENTION
The present invention is in the field of resistance welding pins and method of applying the same and relates more particularly to an improved resistance welding pin used for the attachment of batts or sheets of insulating material to the interior surfaces of air ducts, namely the ducts of air conditioning and heating installations.
2. THE PRIOR ART
It is known to improve the energy efficiency of air conditioning and heating installations employing metallic duct work to apply to the ducts, and normally to the interior portions thereof, batts or sheets of insulation generally one to two inches thick. An adhesive may be used to connect the insulation to the duct. In addition, to assure permanence of attachment, a metallic member having a head portion bearing against the exposed surface of the insulation has a free end portion welded or otherwise attached to the metal duct.
In one form of attachment mode, the attachment member may comprise a pin having an enlarged head and a sharpened tip, the tip being adapted to be connected to the metallic duct by a resistance welding method wherein the duct is connected to a ground electrode and the head of the pin to a live electrode. A current is passed through the length of the pin while the pin is urged toward the duct, resulting in the melting of the tip-adjacent portions of the pin and the tip engaging portions of the duct, whereby the tip of the pin is fused to the duct. The described operation is often carried out through the use of automatic machinery which applies simultaneously a plurality of pins at laterally spaced positions.
A suitable welding pin is disclosed in my U.S. Pat. No. 3,624,340 of Nov. 30, 1971. Apparatus for automatically welding such pin is shown in my U.S. Pat. No. 3,835,285 of Sept. 10, 1974. A machine for the simultaneous attachment of a plurality of pins is shown in U.S. Pat. No. 3,858,024 of Dec. 31, 1974 to Milton Hinden and Charles Giannone. All of the above noted patents are owned by the assignee hereof.
While the pins and attachment apparatuses of the above referenced patents have achieved substantial commercial success, it has been determined that the economy and performance thereof can be improved through the use of a welding pin as hereinafter described and claimed.
More specifically, the welding pins employed heretofore have included an elongate shank having a smooth cylindrical exterior, headed at one end and sharpened at the other. The headed end of the pin has typically been enlarged either integrally or with a separate apertured disk at the under-junction of the head and shank to afford a large contact area with the surface of the relatively fragile insulating material which is usually fabricated of glass fibers. The tip end of the pin is sharpened so as to facilitate penetration through the fibers and provide secure electrical contact with the metal duct.
Since a given installation will include many hundreds or thousands of welding pins, the cost of the pins is a factor to be considered.
The pins heretofore used have employed relatively thick shank portions, in the area of 0.15". As noted in U.S. Pat. No. 3,624,340, the tip angle is pertinent to the effectiveness of the welding procedure. If the tip angle is too acute, the weld may not result in the thickest portion of the shank abutting the surface of the metal duct, with the resultant loss of stability of the attached pin as against side forces exerted thereagainst. If the angle is too obtuse, the tip may not adequately penetrate the fiber mass, and even if such penetration is achieved, due to the relatively low resistance of the tip portion resulting from its thick cross section, rapid melt may not be achieved.
Importantly, as noted hereinabove, it has been found necessary in order to provide both the necessary tip angle and current carrying capacity to form the welding pins with relatively thick shanks even though the strength of the shank, after attachment, is not a factor in the final installation.
Attempts to effect economies through the use of significantly smaller diameter pins, with obvious attendant savings in metal costs, have proved unsatisfactory since such thinner shanks tend to overheat in the course of welding. Where such shanks overheat, the pin body tends to bow or skew due to softening of the metal, whereby the requisite pressures of tip against duct throughout the short cycle of the welding apparatus are not developed.